The invention relates to an X-ray examination means comprising an X-ray tube with at least two different, selectively engageable focal points, comprising a primary radiation diaphragm adjusted relative to the X-ray tube, and comprising an image receiving layer.
It is standard in X-ray technology to design rotating anode tubes in such manner that the anode disk exhibits two focal point paths inclined to different degrees relative to the disk plane. A separate cathode is then allocated to each of said focal point paths. Because the focal point produced on the focal point path which is less inclined relative to the disk plane exhibits smaller axial dimensions with respect to the radial projection direction, i.e., as viewed from the image receiving layer, it is predominantly employed for producing fine-delineating X-ray exposures given simultaneous reduction of its dimensions in the direction perpendicular thereto, this being achieved by means of a corresponding dimensioning of the cathode. However, its maximally admissible dose rate is less than that of the other focal point because of its smaller surface. A larger focal point is likewise generally produced with the cathode assigned to the more greatly inclined focal point path, so that one can also work with a higher dose rate. It is a peculiarity of such X-ray examination means that the two focal points arise at different geometrical locations and switching from the one to the other focal point leads to a maladjustment of the primary radiation diaphragm. Similar to the case of a parallax shift, the switching between focal points results in an admittedly slight but nonetheless noticeable shift of the image area on the image layer plane.
This image shift, of course, becomes all the greater the farther apart the two focal points lie and the smaller the numerical value of the fraction is which derives from the ratio of the spacings focal point/diaphragm to the spacing diaphragm/image layer.